Obesity, which affects 25% of children in affluent societies, is a risk factor for pediatric hypertension, and non-insulin-dependent diabetes, heart disease and early death when it persists into adulthood. Obesity arises because of a mismatch between energy intake and energy expenditure (EE). It is unknown whether this is the result of excess energy intake (ie overfeeding relative to expenditure), or reduced energy expenditure (ie inactivity and/or a suppression in resting EE), or a combination of both. Since the mechanism(s) for this mismatch cannot be resolved in the obese state, studies are proposed using a human, pre-obese model. The hypothesis is that children with low total EE, due to low activity EE (relative to body size) at the onset of the study, will gain more body fat during the subsequent 5 year longitudinal study, after normalization of bodyfat changes due to growth. Total, resting and activity EE over 14 days will be assessed using doubly labeled water and indirect calorimetry, and body composition will be determined using total body water, densitometry, bioelectrical resistance, and, anthropometry. The data will also be used to assess energy requirements in young children. Current information on energy requirements in young children is based on out-dated food intake data, and does not consider variation in body composition or physical activity. These limitations are well recognized and have led to the suggestion that energy requirements should be based on measurement of EE. However, there is a paucity of data on EE in young children. The second aim is to therefore develop and cross- validate equations for predicting energy requirements in young children based on measurement of total EE. The hypothesis is that current estimates of calorie requirements in young children are too high, and simple and accurate equations can be modelled using fat free mass to predict resting energy requirements, and heart rate as a marker of physical activity. These studies will provide important information on the role of BE and physical activity in the development of childhood obesity, and on energy requirements in young children. The results will enable identification of children at highest risk for developing obesity and will form the groundwork for future work to design intervention strategies using intake and exercise prescriptions to prevent obesity in young children.